1. Field of the Invention
This invention relates to stirrers for metallurgical melts and more particularly to mechanical stirrers for metallurgical melts.
2. Description of the Prior Art
In metallurgical melts it is desirable to obtain a homogenous mixture of the various reaction constituents to provide a consistent high quality product. To obtain the desired homogenity various means of agitation have been considered which aid in the intermixing of reaction constituents. Among these agitation means are mechanical stirrers which when rotated provide a pumping action thus agitating the melt. An early apparatus to accomplish the pumping action within the melt has a vertical pipe, open at the top and closed at its bottom with horizontal open pipes connected to the vertical pipe. The horizontal pipe openings are in communication with the inner opening of the vertical pipe. This apparatus was rotated at high speed and additions of solid material were made to the melt through the vertical pipe's top opening.
Advances in metallurgy, especially ferrous metallurgy, have combined to subject metallurgical melts, to which solid, liquid, or gaseous additions may be made, to rapid reactions of various kinds and to severe restrictions in analysis and resultant characteristics of final product and have created an increased need for improved mixing and treating means. Even though, for example, the hot metal or molten steel from an open hearth or basic-oxygen furnace has been refined to a degree generally experienced in only recent years, further refining can be accomplished by the addition of agents to the molten metal. A brief opportunity for such treatment is provided in the hot metal ladle, to the contents of which refining elements can be added, either to the surface or by means of stirrers or mixers into the interior thereof.
The stirrer of the present invention meets this need for rapid and effective refining of hot metal; the need is further demonstrated by the number of prior art efforts, exemplified by the following items of prior art, that sought unsuccessfully or inadequately to meet such a need.
The centrifugal pumping action obtainable by rotating a body having a fluid inlet near its axis and connecting channels to its periphery has long been known as shown, for example, in MacNeille U.S. Pat. No. 1,986,836, issued Jan. 8, 1935. In Heuer U.S. Pat. No. 2,290,961, issued July 28, 1942 such a rotating body with axial inlet and ascending connecting channels is employed as the impeller of desulphurizing apparatus for the treatment of molten iron; but the shape of the impeller especially in respect of the upper surface above the channel exits is substantially that of a paddle. Rotation of this paddle will create a vortex and cause the slag phase on the surface of the melt to be pushed to the edges of the vessel impairing the function of these agents. In addition the paddle effect will soon impel the entire body of the melt to circulate in the vortex thus minimizing the mixing of agents within the melt. Another early form of stirrer appeared in the "Foundry Trade Journal" of Feb. 25, 1940. The apparatus therein shown did, owing to its exterior surfaces, exert a lot of friction upon the melt and as a consequence produces and unfavorable pressure-pattern in the melt, and it furthermore makes no provision for exchange of material between eddies in the melt.
In Sundstrom et al. U.S. Pat. No. 2,525,973, issued Oct. 17, 1950 the feeding of a treating agent into and beneath the surface of molten iron contemplates such addition to the pig-iron runner and further exemplifies the technological demand to add refining agents to molten metal and in rapid intimate mixture therewith. Another feeding apparatus is that of Clenny et al. U.S. Pat. No. 285,812, issued Oct. 28, 1958 which employs gas under pressure to feed refining agents into and beneath the surface of molten metal. Neither of the devices provides means for maintaining thorough mixing of the agents with the molten metal.
Ostberg et al. U.S. Pat. No. 3,278,295, issued Oct. 11, 1966 discloses and claims a method of stirring a metal with a vertical-pipe device which demonstrates again the paddle effect of excessive melt rotation. Exemplary of such a stirrer is also German patent No. 1,190,479. The Ostberg et al. patent claims only a path of movement in a greater portion of the charge and an additional path through an elongated vertical feed pipe and lateral pipes of at least 15 percent of the sum of the height and diameter of the charge. Such an elongated pipe would interfere, as hereinafter shown, with unexpected improvements of the subject matter of invention, and was thought to be necessary when stirring effects in the bath were imperfectly understood. Another apparatus of which the impeller, here adapted for feed of agents into the molten steel, is essentially a paddle on the end of a pipe is that disclosed in Japanese patent No. 47-51681 which issued Mar. 3, 1969. Inert gas is fed in with the agents to give some agitating (mixing) action to the molten steel, but otherwise the stirring provided effects only the usual vortical movement above the central axis.
A Russian patent, No. 280,505, issued Dec. 8, 1970 displays a mixer, having a long vertical "collector tube" similar to that of the aforementioned Ostberg et al. which mixer acts as a centrifugal pump to move hot metal out of side vents onto the slag bed of hot metal in the ladle. The Russian apparatus effects thus a different purpose, and in fact, were it sufficiently immersed to act on the surrounding metal, having a flat top configuration, would have the usual paddle effect.
Ostberg U.S. Pat. No. 3,573,895 issued Apr. 6, 1971, introduces gas at the junction point of the vertical pipe shaft and lateral arms to bring up flowing hot metal into contact with the slag layer on a body of hot metal, and neither contemplates nor provides for mixing of agents in the said body of metal.
Polomsky, Canadian Patent No. 871,006, which issued May 18, 1971 to Demag, A. G. discloses another method where extreme metallurgical results are aimed at by injecting agents into the molten metal in a vessel such as hot metal ladle. This patent expressly notes that the device employed induces a circulatory flow in the metal, and, in fact, the apparatus with its sidearms laterally protruding is operated fully submerged in the melt where the sidearms force the surrounding melt to co-rotate almost synchronously. This not only requires much power but also because of the shape of the sidearms causes splashing; and, in addition the substantially synchronous rotation of the melt with the rotation of the arms makes homogenization of the injected material too slow.
The metallurgy associated with the treatment of steel melts, the role of the basic slag layer, and the chemistry of additives injected into the melt are described at length in Richter et al. U.S. Pat. No. 3,885,957 issued May 27, 1975 in which also is taught the critical value of the depth of addition of the additives under the given circumstances, i.e., over 2000 mm. below the surface of the melt. In Richter et al. no special provision is made for improving the mixing of the additives.
Furck et al., U.S. Pat. No. 3,887,172, issued June 3, 1975 and discloses the treatment of molten metals in a casting ladle by introducing gas in a rising column of molten metal in a vertically entered conduit; while the claimed apparatus effects no mechanical pumping action at all, the patent clearly describes, under Background of the Invention, the techniques that had theretofore been applied for such metal treatment and the numerous difficulties, e.g. disruption of the slag layer, that had been encountered with known mechanical devices.
A disadvantage created by a surface vortex on the surface of stirred molten metal that is in addition to disturbing the layer of reaction slag is described in Downing et al. U.S. Pat. No. 4,063,932 which issued Dec. 20, 1977. This patent, however, shows only a simple stirrer without means for mechanically pumping molten metal or additives into the melt.
The foregoing references demonstrate the efforts expended by industry for several decades to accomplish rapid and efficient mixing of additives such as desulfurizing or alloying agents, many of which incidentally, are of low specific gravity, into molten metal, especially molten steel.
In accordance with the present invention a mechanical stirrer for metallurgical melts now is provided which is designed for maximum mixing efficiency through pumping, is adapted for the introduction of fine grained material into the melt, and adapted for collecting liberated gas. The liberated gas is not only collected by the apparatus, as when gas is produced by the reaction of additives with the melt, but the apparatus also provides for a zone of low pressure within the melt whereby a degassing of the melt is obtained.
Further advantages provided by the invention will be apparent from the following description thereof by way of the following drawings in which like elements are identified by like reference numerals.